Don't run deploy-docs-preview if the event does not have PRs (#3047)

Author: DouweM

.github/workflows/after-ci.yml

@@ -41,7 +41,7 @@ jobs:
 
   deploy-docs-preview:
     runs-on: ubuntu-latest
-    if: github.event.workflow_run.event == 'pull_request' && github.event.workflow_run.head_branch != 'main'
+    if: github.event.workflow_run.event == 'pull_request' && github.event.workflow_run.pull_requests[0] != null
     environment:
       name: deploy-docs-preview